The present invention relates generally to a draft device for a bubble column and, more specifically a bubble column having plural plates. A bubble column is a reaction vessel which is generally used for a reaction wherein gas and liquid are mixed, particularly for fermentation, and which is supplied at its bottom with a gas such as air for causing agitation therein.
A bubble column has a number of advantages over a reaction vessel having an agitator, such as lower cost of installation, easier maintenance and a higher efficiency of agitation per unit power. However, disadvantageously, the liquid in the column is circulated at a low rate, and the bubbles formed in the column are difficult to remove.
As an improved bubble column, there has been proposed a bubble column having a draft tube of cylindrical construction which is coaxial with the column, as disclosed in Japanese Published Unexamined Patent Application No. 99380/1973. In such device, either the outside or inside of the draft tube is aerated or supplied with a gas, and after the gas is caused to flow upward in the aerated area, it flows downward through the non-aerated area, so that the agitation of the liquid in the column may be enhanced. The formed bubbles are entrained in the down flow and disappear.
However, in some reaction procedures, it is necessary to supply a large quantity of culture medium as the reaction proceeds. When a bubble column is used for such reaction, the level of the liquid in the column changes as its quantity increases. For reactions of this type, there has been proposed a bubble column having a draft tube of which the height is adjustable to maintain the same effect of agitation when the quantity of the liquid in the column is changed as is disclosed in Japanese Published Unexamined Patent Application No. 55780/1979. In this known device, a complicated arrangement is, however, required for varying the height of the draft tube during the reaction. It is, therefore, desirable to develop a simple device which provides an effective liquid circulation in a bubble column. To this end, it has been found that a draft device having a plurality of plates provided in a bubble column enables efficient circulation of the liquid in the column and substantial gas flow entraining bubbles in addition to the above-mentioned effect of a draft tube.